This invention relates to high performance audio systems, and more particularly to multi-amplifier audio systems for vehicular installations.
Car stereo systems face unique problems in high fidelity reproduction of recorded or broadcast sound, because speaker placement, speaker types, amplifier power, crossover networks, limited internal space, internal vehicle geometry, and other factors can all affect the quality and characteristics of the sound which the listener hears. Increasing amplifier power, despite the consequent expense, does not confront the major problems, which derive both from the limited space available for installations and the complex nature of internal reflections within a vehicle. Acoustic waves launched from a given speaker location into the interior of a vehicle are reflected within relatively short distances off interior surfaces. They then will often reflect back and forth between opposed surfaces to establish standing waves, thus creating resonance peaks within the audible frequency spectrum. Because the interior dimensions of a vehicle are limited, resonances arise in the longer wave (low frequency) region of approximately 60 Hz or less to 300 Hz or more. Moreover, such simple resonances are often accompanied by complex standing waves which are created because of multiple, oblique-angled reflections off different surfaces within the three-dimensional volume of the vehicle.
For many years car stereos were designed for the younger market, and these car stereo users and perhaps also the music they preferred in many instances created a demand for the "boomy" bass characteristic inherent in low frequency resonances. When greater discrimination began to be exercised, systems were augmented with graphic equalizers by which the frequency spectrum could be subdivided into multiple bands (typically from about 3 to 12 separate bands) and each could be adjusted in amplitude. This approach allows for some specific adjustments but has been predominantly used with single amplifier types of systems.
There has been an increasing recent trend toward improving the fidelity of car audio systems, as opposed to earlier tendencies to use excessive power at low frequency levels. An earlier stereo installation might have used two speakers, each comprising a mid-range and tweeter unit, spaced apart in the front or rear of the vehicle. These would be driven through a crossover network from a single amplifier. It is now common, however, to use "multi-amp" installations, in which speakers for the different frequency ranges are each driven by a separate amplifier. The value of cleaner low frequency ranges has become more apparent and separately driven woofers and sub-woofers are thus increasingly being used. The multi-amp installations include so-called "biamps", employing a two-way division of the frequency ranges, and "tri-amps" in which the division is between low frequency (woofer), mid-range unit and high frequency (tweeter). A subwoofer is often alternatively used for the lowest frequency range to enhance bass response, the sub-woofer unit often being monaural.
To achieve a substantially flat frequency response within a vehicle using a multi-amp system, the trend has been to use electronic crossovers. The electronic crossovers are adjustable as to crossover point, and operate more efficiently than do passive crossover networks. Because they are adjustable, a troublesome resonance or null in a given frequency range can be compensated by spacing crossover points so as to diminish response, or overlapping the crossover points so as to enhance response.
Known electronic crossover systems are limited in their capabilities, as presently implemented, because they are generally restricted to two separate independently adjustable frequency bands. It is recognized that they can be cascaded (used in series) to give tri-amp as well as bi-amp capability, but this limits the capability for adjustment because a later crossover can only choose a higher high-pass (or a lower lowpass) level for cutoff.
When a multi-amp system is installed in a vehicle, the number and placement of speakers, and the number and placement of the electronic circuits, are determined by the spaces available. The internal geometry of the vehicle can vary with car style, even in a particular model (e.g. two door vs. four door) and with the interior materials that are used. Thus if electronic crossovers are to be used to flatten frequency response, or shape frequency response to the preferences of the listener, a design having novel versatility is required. A high degree of pegmentation of the frequency band may be used in accordance with the performance of a specific combination of units in a particular vehicle. Additionally, it may be desired to revise an existing system, as by adding new components to convert from a bi-amp to a tri-amp system. Applications need not, of course, be limited to the conventional tweeter, mid-range, and woofer or sub-woofer configurations, inasmuch as it may be desirable for some installations to utilize a five-way or seven-way arrangement or even greater number of speakers, together with varying crossover points and different speakers ranging from super tweeters to sub-woofers.